Closest Star Proxima Centauri Has a Dust Belt, Maybe More Planets

The closest star to us, Proxima Centauri, and the planet that orbits it, Proxima b, are just 4.2 light-years away. The cool and faint red dwarf star is part of the Alpha Centauri system along with two larger stars. The planet Proxima b thrilled the astronomy community when it was discovered in August 2016, being not only the closest exoplanet to the solar system, but also terrestrial and the size of Earth. Even better, if the planet has an atmosphere, it could have surface temperatures of 86 degrees Fahrenheit (30º C) and possibly liquid water.

A new study has revealed that Proxima b is not the only thing that orbits our closest stellar neighbor. Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile detected emissions from a dust belt around the small star. Similar to the asteroid belt in our own solar system, the ring of dust around Proxima Centauri could have particles ranging from dust grains less than a millimeter to asteroids that stretch multiple kilometers wide. The belt of cool dust, estimated at negative 382 degrees Farenheit (-230º C), is positioned one to four times as far from Proxima Centauri as the Earth is from the sun.

"The dust around Proxima is important because, following the discovery of the terrestrial planet Proxima b, it's the first indication of the presence of an elaborate planetary system, and not just a single planet, around the star closest to our sun," says lead author of the new study, Guillem Anglada of the Instituto de Astrofísica de Andalucía (CSIC) in Spain.

The ALMA data also suggests there may be another, fainter belt of dust about 10 times farther from Proxima Centauri than the inner ring. The outer ring, if it exists, would be an exciting feature for astronomers to study due to its frigid temperatures so far away from its small host star. These dust belts are the leftover debris that did not aggregate into planets, suggesting other fully-formed planets may be lurking in the system yet undiscovered. The findings are to be published in the Astrophysical Journal Letters.

"This result suggests that Proxima Centauri may have a multiple planet system with a rich history of interactions that resulted in the formation of a dust belt. Further study may also provide information that might point to the locations of as yet unidentified additional planets," said Anglada in a press release.

The Proxima Centauri system can be understood as a miniature version of our solar system. The red dwarf star is only about 1/7 the diameter of the sun, or about 1.5 times that of Jupiter. Its one confirmed planet, Proxima b, orbits much closer to the star than the dust belt, at just 2.5 million miles (4 million kilometers), which is only about 0.03 AU. The inner and outer dust belts of Proxima Centauri bear similarities to the asteroid belt between Mars and Jupiter and the Kuiper belt out beyond the orbit of Pluto.

This image of the sky around the bright star Alpha Centauri AB also shows the much fainter red dwarf star, Proxima Centauri, the closest star to the Solar System.

Digitized Sky Survey 2/Davide De Martin/Mahdi Zamani

"These first results show that ALMA can detect dust structures orbiting around Proxima. Further observations will give us a more detailed picture of Proxima's planetary system," says co-author of the study Pedro Amado, also from the Instituto de Astrofísica de Andalucía. "In combination with the study of protoplanetary discs around young stars, many of the details of the processes that led to the formation of the Earth and the solar system about 4600 million years ago will be unveiled. What we are seeing now is just the appetiser compared to what is coming."

If other planets orbit Proxima Centauri, it is only a matter of time before astronomers pinpoint them and measure their properties. Could life exist in this intriguing planetary system? It's difficult to say. Proxima b is likely tidally locked with its star, meaning one side always faces the light of Proxima Centauri and the other side is always in darkness. Planets around red dwarfs could be a good bet for finding life because the stars live for so long—in fact, astronomers believe red dwarfs have longer lifespans than the current age of the universe, about 13.8 billion years—giving life plenty of time to take root. However, these types of stars are also known to be quite volatile in their early lives, ejecting supercharged particles from surface eruptions that might strip any orbiting planets of their insulating atmospheres.

This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope.

Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani

In addition to follow-up observations with ground-based telescopes, a group of astronomers is working to send a small spacecraft—a computer chip with a small camera, no bigger than a floppy disk—all the way to Proxima Centauri using light sails propelled by Earth-based lasers. The program, known as Breakthrough Starshot, could possibly send a nanoprobe on the 4.2-light-year journey in just 20 or 30 years.

The closest star to us has an Earth-sized planet. Another nearby red dwarf star, TRAPPIST-1, has seven. The Kepler space telescope finds new exoplanets seemingly every day. And now we know Proxima Centauri has a dust belt, possibly two, that suggest an elaborate planetary system. The hunt for Earth's twin continues, and you can't help but think we're getting closer.

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